This invention relates to internal combustion engines of the type in which a piston reciprocates in a cylinder. Piston internal combustion engines are generally of types known as two-stroke engines and four-stroke engines. A four-stroke engine has four separate strokes of the piston within the cylinder for each cycle. These are known as the intake stroke in which a mixture of air and fuel is drawn into the cylinder, the compression stroke in which this combustible mixture is compressed, the power stroke which is the stroke in which the piston is forced away from the head of the cylinder by the energy of the burning air-fuel mixture, and the exhaust stroke in which the piston moves toward the cylinder head and pushes the burned combustion products from the cylinder through an exhaust valve. A two-stroke engine has only two strokes per cycle. In a two-stroke engine the exhaust function and the intake function occur simultaneously at the end of the power stroke, as will be discussed.
The power output of an internal combustion engine is normally controlled by throttling the air-fuel supply. When an internal combustion engine is running at constant speed with the throttle approximately fully opened, it will function smoothly and efficiently. However, when the speed is varied and particularly when the air-fuel mixture is throttled to a great extent, an internal combustion engine will run poorly and inefficiently.
In a two-stroke engine the exhaust function is usually accomplished by providing ports in the cylinder wall that are uncovered when the piston is at the end of the power stroke. When the exhaust ports are uncovered high pressure exhaust gas escapes through an exhaust manifold, however, the cylinder is full of residual exhaust gas at approximately atmospheric pressure when the compression stroke begins. While the exhaust is being vented, the combustible mixture of air and fuel is pumped into the cylinder, usually by crankcase pressure that is generated by the underside of the piston during the power stroke. Venting exhaust and introducing air-fuel mixture simultaneously is done to a greater or lesser degree of efficiency by directing the flow of air-fuel mixture to flush out exhaust. Since air-fuel mixture is forced into the cylinder by the pressure in the crankcase, it must enter the cylinder at the end of the power stroke because crankcase pressure diminishes during the compression stroke because of movement of the piston toward the cylinder head and away from the crankcase. When the compression stroke is complete the cylinder contains both the air-fuel mixture and residual exhaust gas.
When a two-stroke engine is running with the throttle open it functions adequately because the proportion of air-fuel mixture to exhaust gas in the cylinder is high. However, when running under throttled conditions, the proportion of exhaust gas is so high that its dilution effect on the air-fuel mixture prevents combustion. As a result, the cylinder doesn't fire until one or more further cycles are completed because each cycle without firing increases the proportion of air-fuel mixture to exhaust, so eventually there is enough combustible gas in the cylinder to fire. This phenomenon produces the characteristic sound of two-stroke engines at idling speeds or at low speeds. However, even when a two-stroke engine is running with the throttle open enough to provide smooth firing, combustion is suppressed by the presence of exhaust gas to a greater or lesser extent and the full value of the fuel is not obtained with a resultant loss of efficiency.
Four-stroke internal combustion engines also are made to run most effectively with an open throttle. The efficiency of a four-stroke engine is related to the pressure in the cylinder at the end of the compression stroke. When functioning at low operating speeds with the throttle restricted to limit the amount of air-fuel mixture taken into the cylinder, at the end of the intake stroke the internal pressure of the cylinder is much lower than it would be if the engine were running with the throttle open. As a result, the pressure in the cylinder at the end of the compression stroke is too low for efficient operation.
It has been suggested in the past to deal with this problem by introducing exhaust gas into the cylinder of a four-stroke internal combustion engine. This has been accomplished by having the piston uncover ports in the cylinder that are open to exhaust when the piston is at the end of the intake stroke, for example, as illustrated in U.S. Pat. No. 3,583,375, or by keeping the exhaust valve open through some portion of the intake stroke to draw exhaust already expelled from the cylinder back into it. In all such arrangements, the dilution of the incoming air-fuel mixture with exhaust gas influences the combustibility of the air-fuel mixture. In extreme cases, dilution of the air-fuel mixture with exhaust will cause the gas mixture in the cylinder to be incombustible and in less extreme cases, the presence of combustion products in the cylinder suppresses the ability of the air-fuel mixture to burn effectively.